Liquid crystal polyester fiber and producing method thereof

ABSTRACT

Provided is a liquid crystal polyester fiber having high strength, high elastic modulus, high abrasion resistance, excellent processability, and little thermal deformation at high temperature, and also provided is a production method thereof. A liquid crystal polyester fiber, characterized in that the peak half-value width of the endothermic peak (Tm1) observed when measuring by differential calorimetry under rising temperature conditions starting at 50° C. and increasing 20° C./min is 15° C. or higher, the polystyrene-converted weight-average molecular weight is between 250,000 and 2,000,000 inclusive, the peak temperature of the loss tangent (tan δ) is between 100° C. and 200° C. inclusive, and the peak value of the loss tangent (tan δ) is between 0.060 and 0.090 inclusive. A mesh fabric comprising the liquid crystal polyester fiber. A production method for melt liquid crystal polyester fiber, characterized in that liquid crystal polyester fiber obtained by melt-spinning is subject to solid-phase polymerization, and subsequently heat treated at a stretch ratio of at least 0.1% and under 3.0% at a temperature at least 50° C. higher than the endothermic peak temperature (Tm1) as observed when measuring by differential calorimetry under rising temperature conditions starting at 50° C. and increasing 20° C./min.

TECHNICAL FIELD OF THE INVENTION

Our invention relates to a liquid crystal polyester fiber having high strength, high elastic modulus, high abrasion resistance, excellent processability and less heat deformation at a high temperature, and a manufacturing method thereof.

BACKGROUND ART OF THE INVENTION

A liquid crystal polyester is a polymer consisting of rigid molecular chains, showing high strength and high elastic modulus among fibers produced in a melt spinning process by applying a heat treatment (solid-phase polymerization) to the molecular chains highly-oriented in a fiber axial direction. As shown in pages 235-256 of Non-patent document 1, the liquid crystal polyester has improved heat resistance and dimensional stability since the solid-phase polymerization increases its molecular weight to raise its melting point. Thus the liquid crystal polyester fiber has high strength, high elastic modulus, excellent heat resistance and excellent thermal dimensional stability by applying the solid-phase polymerization.

On the other hand, the liquid crystal polyester fiber may have disadvantages such as low interaction in a fiber axial direction and poor abrasion resistance so that fibrillation is caused by frictions in higher processing and weaving process, because rigid molecular chains are highly oriented in the fiber axial direction to form dense crystals. Recently, specifically for filters and screen-printing gauzes made of monofilaments, higher weaving density (higher mesh) and larger opening section areas are demanded in order to improve the performance. Since improvements such as higher single fiber fineness, higher strength and higher elastic modulus are strongly demanded to achieve this, the liquid crystal polyester fiber is being counted on because of its high strength and high elastic modulus. Since the fault decrease in fibril or the like is also strongly demanded for higher performance at the same time, improvements of abrasion resistance of the liquid crystal polyester fiber and processability are expected.

Further, thermal deformation should be less even at a high temperature for mesh fabric products. For example, a great thermal deformation at a high temperature with high load for reducing wrinkles might cause non-uniform openings and degrade performances of screen printing and filtration. From these viewpoints, it is demanded for the liquid crystal polyester fiber to improve abrasion resistance and suppress thermal deformation at a high temperature at the same time.

In order to improve abrasion resistance of liquid crystal polyester fiber, pages 18-19 of Patent document 1 suggest that liquid crystal polyester fiber should be heat-treated at the melting point+10° C. or more, or the endothermic peak temperature (Tm1)+10° C. or more, wherein the Tm1 is determined by differential calorimetry under temperature elevation of 20° C./min from 50° C. Although that technology can improve the abrasion resistance well, it cannot sufficiently suppress the thermal deformation at a high temperature. The great improvement of abrasion resistance is likely to increase the thermal deformation at a high temperature although fiber after the solid-phase polymerization is heat treated at a high temperature to improve the abrasion resistance in Patent document 1. Therefore, the technology disclosed in Patent document 1 by itself cannot achieve both abrasion resistance improvement and thermal deformation suppression at a high temperature.

Patent document 1 doesn't disclose any suggestion of suppressing thermal deformation at a high temperature, as only disclosing running stability in page 20 describing the change of elongation ratio from 2%-relaxation rate to 10%-stretch rate about high-temperature heat treatment of liquid crystal polyester fiber after solid-phase polymerization. It doesn't even disclose any suggestion of advantage of a guide provided after the heat treatment with respect to the running stability for the heat treatment.

Page 2 of Patent document 2 discloses a technology in which liquid crystal polyester fiber after solid-phase polymerization is subject to a thermal stretch by 10% or more as a high-temperature heat treatment. However, Patent document 2 doesn't disclose any suggestion to suppress a thermal deformation at a high temperature, as only disclosing the purpose of the stretch, such as abrasion resistance improvement and thinning by stretching fiber.

Page 15 of Patent document 3 discloses a technology to thermally stretch the liquid crystal polyester fiber before solid-phase polymerization by less than 1.005 ratio. With this technology, the liquid crystal polyester fiber is stretched before the solid-phase polymerization at a relatively low temperature of the glass transition temperature+50° C. or less, while it discloses neither the improvement of abrasion resistance by the heat treatment at a high temperature of the melting point+50° C. or more nor the suggestion about thermal deformation at the high temperature. Although Patent document 3 discloses a dynamic viscoelastic measurement of tan δ to obtain Tg (glass transition temperature) of the resin, it doesn't disclose any relation between tan δ and thermal deformation suppression at a high temperature.

Page 2 of Patent document 4 discloses a technology of solid-phase polymerization (heat treatment) of liquid crystal polyester fiber performed at a temperature of Tm−80° C. or less, and subsequently at another temperature between Tm−60° C. and Tm+20° C. With this technology, the temperature for solid-phase polymerization is raised stepwise to improve a vibration damping characteristics, while it discloses neither the improvement of abrasion resistance by the heat treatment at a high temperature of the melting point+50° C. or more nor the suggestion about thermal deformation at the high temperature. Although Patent document 4 discloses tan δ measured as an index to represent vibration damping characteristics of solid-phase polymerized liquid crystal polyester fiber, it doesn't disclose any relation between tan δ of liquid crystal polyester fiber prepared by a high-temperature heat treatment at the melting point+50° C. or more and thermal deformation suppression at a high temperature.

PRIOR ART DOCUMENTS Patent Documents

-   Patent document 1: JP2008-240230-A -   Patent document 2: JP2010-189819-A -   Patent document 3: JP2006-89903-A -   Patent document 4: JP-H4-289218-A

Non-Patent Documents

-   Non-patent document 1: “Reforming technology and the latest     applications of liquid crystal polymer”, edited by Technical     information institute, Co., Ltd, pp. 235-256(2006)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As an object of our invention, it could be helpful to provide a liquid crystal polyester fiber having high strength, high elastic modulus, high abrasion resistance, excellent processability and less heat deformation at a high temperature, and a manufacturing method thereof.

Means for Solving the Problems

The above-described object of our invention can be achieved by the following means.

(1) A liquid crystal polyester fiber having: a peak half-value width of 15° C. or more at an endothermic peak (Tm1) observed by a differential calorimetry under a temperature elevation condition of 20° C./min from 50° C.; a weight-average molecular weight in terms of polystyrene of 250,000 or more and 2,000,000 or less; a peak temperature of a loss tangent (tan δ) of 100° C. or more and 200° C. or less; and a peak value of the loss tangent (tan δ) of 0.060 or more and 0.090 or less. (2) A mesh fabric comprising the liquid crystal polyester fiber of (1). (3) A producing method of a melt-spun liquid crystal polyester fiber characterized in that a liquid crystal polyester fiber made by a melt spinning is polymerized in a solid phase and then heated at a temperature of an endothermic peak (Tm1)+50° C. or more by a stretch rate of 0.1% or more and less than 3.0%, wherein the endothermic peak is observed by a differential calorimetry under a temperature elevation condition of 20° C./min from 50° C.

Effect According to the Invention

Our liquid crystal polyester fiber can be excellent in abrasion resistance and processability, so that the weaving performance in producing a product such as mesh fabric is enhanced to reduce faults in the product. Further, it has a small thermal deformation even at a high temperature, so that a fabric product has only a small variation in performance and dimension through the high-temperature treatment. Furthermore, the producing method of our invention can produce the liquid crystal polyester fiber efficiently.

Embodiments for Carrying Out the Invention

Hereinafter, our liquid crystal polyester fiber will be explained in details.

The liquid crystal polyester described in the specification means a polyester capable of forming an anisotropic melting phase (liquid crystallinity) when molten. This characteristic can be recognized by observing light transmitted through the sample under polarized radiation when a sample of liquid crystal polyester is placed on a hot stage and heated in nitrogen atmosphere, for example.

The liquid crystal polyester in the specification may be:

a) a polymer of an aromatic oxycarboxylic acid component;

b) a polymer of an aromatic dicarboxylic acid component, an aromatic diol component and/or an aliphatic diol component; and

c) a copolymer of a) and b).

It is preferable that the liquid crystal polyester is a wholly aromatic polyester prepared without the aliphatic diol component for achieving high strength, high elastic modulus and high heat resistance. The aromatic oxycarboxylic acid component may be an aromatic oxycarboxylic acid such as hydroxy benzoic acid and hydroxy naphthoic acid, and may be alkyl, alkoxy or halogen substitution product of the aromatic oxycarboxylic acid. The aromatic dicarboxylic acid component may be an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, diphenyl dicarboxylic acid, naphthalene dicarboxylic acid, diphenylether dicarboxylic acid, diphenoxyethane dicarboxylic acid and diphenylethane dicarboxylic acid, and may be alkyl, alkoxy or halogen substitution product of the aromatic dicarboxylic acid. The aromatic diol component may be an aromatic diol component such as hydroquinone, resorcinol, dioxydiphenyl and naphthalene diol, and may be alkyl, alkoxy or halogen substitution product of the aromatic diol. The aliphatic diol component may be an aliphatic diol such as ethylene glycol, propylene glycol, butane diol and neopentyl glycol.

It is preferable that the liquid crystal polyester is a copolymer of p-hydroxy benzoic acid component, 4,4′-dihydroxy biphenyl component, hydroquinone component, terephthalic acid component and/or isophthalic acid component, a copolymer of p-hydroxy benzoic acid component and 6-hydroxy 2-naphthoic acid component, a copolymer of p-hydroxy benzoic acid component, 6-hydroxy 2-naphthoic acid component, hydroquinone component and terephthalic acid component or the like, for achieving excellent spinnability, high strength, high elastic modulus, and abrasion resistance improved by high-temperature heat treatment after solid-phase polymerization.

It is preferable that the liquid crystal polyester comprises the following structural units (I), (II), (III), (IV) and (V). Besides, “structural unit” means a unit capable of composing repeated structures in a main chain of polymer in the specification.

This combination of structural units makes it possible for the molecular chain to have a proper crystallinity and a non-linearity, namely, a melting point capable of being melt spun. Therefore a good yarn-making property can be exhibited at a spinning temperature set between the melting point and the thermal decomposition temperature of polymer, as providing fiber uniform along the longitudinal direction, while the strength and elastic modulus of fiber can be enhanced with appropriate crystallinity.

Further, it is important to combine components of diol with a high linearity and such a small bulk as structural units (II) and (III), so that the molecular chain in the fiber can have an orderly structure with less disorder while the crystallinity does not increase excessively and the interaction in a direction perpendicular to the fiber axis can be maintained. In addition to obtaining high strength and elastic modulus as such, particularly excellent abrasion resistance can be achieved by carrying out a heat treatment at a high temperature after solid-phase polymerization.

It is preferable the structural unit (I) is contained by 40 to 85 mol %, more preferably 65 to 80 mol %, further preferably 68 to 75 mol %, in total of structural units (I), (II) and (III). By setting the content in such a range, the crystallinity can be controlled properly, high strength and elastic modulus can be achieved while the melting point can be controlled in a range suitable for performing a melt spinning.

It is preferable that the structural unit (II) is contained by 60 to 90 mol %, more preferably 60 to 80 mol %, further preferably 65 to 75 mol % in total of structural units (II) and (III). By setting the content in such a range, since the crystallinity does not increase excessively and the interaction in a direction perpendicular to the fiber axis can be maintained, the abrasion resistance can be improved by carrying out a heat treatment at a high temperature after solid-phase polymerization.

It is preferable that the structural unit (IV) is contained by 40 to 95 mol %, more preferably 50 to 90 mol %, further preferably 60 to 85 mol % in total of structural units (IV) and (V). By setting the content in such a range, the melting point of the polymer can be controlled properly, a good spinnability can be exhibited at a spinning temperature set between the melting point and the thermal decomposition temperature of the polymer, so that fiber uniform along the longitudinal direction is prepared. Further, since the linearity of the molecular chain loosens appropriately, the abrasion resistance can be improved while the interaction in a direction perpendicular to the fiber axis can be enhanced with a fluctuant fibril structure by carrying out a heat treatment at a high temperature after solid-phase polymerization.

Preferred ranges of the respective structural units of the liquid crystal polyester are as follow. Desirable liquid crystal polyester fiber can be obtained by controlling the composition in these ranges so as to satisfy the above-described condition.

Structural unit (I): 45-65 mol % Structural unit (II): 12-18 mol % Structural unit (III): 3-10 mol % Structural unit (IV): 5-20 mol % Structural unit (V): 2-15 mol %

In addition to the above-described structural units, it is possible to copolymerize an aromatic dicarboxylic acid such as 3,3′-diphenyl dicarboxylic acid and 2,2′-diphenyl dicarboxylic acid, an aliphatic dicarboxylic acid such as adipic acid, azelaic acid, sebacic acid and dodecanedionic acid, an alicyclic dicarboxylic acid such as hexahydro terephthalic acid (1,4-cyclohexane dicarboxylic acid), an aromatic diol such as chloro hydroquinone, 4,4′-dihydroxy phenylsulfone, 4,4′-dihydroxy diphenylsulfide and 4,4′-dihydroxy benzophenone, p-aminophenol or the like, in the liquid crystal polyester by 5 mol % or less as far as advantages of our invention are achieved.

It is possible to add a polyester, a vinyl-based polymer such as polyolefin and polystyrene, or another polymer such as polycarbonate, polyamide, polyimide, polyphenylene sulfide, polyphenylene oxide, polysulfone, aromatic polyketone, aliphatic polyketone, semi-aromatic polyester amide, polyetheretherketone and fluororesin. It is preferable to add polyphenylene sulfide, polyetheretherketone, nylon 6, nylon 66, nylon 46, nylon 6T, nylon 9T, polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polycyclohexane dimethanol terephthalate, polyester 99M or the like. From a viewpoint of good yarn-making property, it is preferable that such a polymer has a melting point within a range of the melting point of the liquid crystal polyester ±30° C.

It is possible to add a small amount of an inorganic substance such as various metal oxides, kaoline and silica or an additive such as colorant, delustering agent, flame retardant, anti-oxidant, ultraviolet ray absorbent, infrared ray absorbent, crystal nucleus agent, fluorescent whitening agent, end-group closing agent and compatibilizing agent as far as advantages of our invention are achieved.

The liquid crystal polyester fiber should have a weight average molecular weight (may be called merely “molecular weight”) of 250,000 to 2,000,000 in terms of polystyrene. The high molecular weight of 250,000 or more contributes to high strength, elastic modulus and elongation. Because the strength, elastic modulus and elongation are likely to increase as the molecular weight becomes higher, it is preferable that the molecular weight is 300,000 or more, preferably 350,000 or more. The upper limit of molecular weight may be around 2,000,000 and may be sufficient at 1,000,000. The molecular weight is determined by the method to be explained in the Example.

The liquid crystal polyester fiber should have 15° C. or higher of peak half-value width observed by differential calorimetry under temperature elevation condition of 20° C./min from 50° C. Tm1 in this determination method represents a melting point of fiber. The wider the area of the peak shape is, or the greater the heat of melting (ΔHm1) is, the higher the crystallinity is. Also the smaller the half-value width is, the higher the completeness of crystal is. By melt-spinning and then polymerizing the liquid crystal polyester in a solid-phase, Tm1 elevates, ΔHm1 increases and the half-value width decreases, and the crystallinity and completeness of crystal increases, so that the fiber increases in strength, elongation and elastic modulus as improving in heat resistance. On the other hand, the abrasion resistance deteriorates, probably because a difference in structure between the crystal part and the amorphous part becomes remarkable by increase of the completeness of crystal so that destruction occurs in the interface therebetween. Accordingly, while maintaining high Tm1 as well as high strength, elastic modulus, elongation and heat resistance observed in fiber which has been polymerized in a solid-phase, the crystallinity of our fiber is decreased by increasing the peak half-value width above 15° C. observed in liquid crystal polyester fiber without solid-phase polymerization, so that the abrasion resistance can be improved by decreasing the difference in structure between the crystal/amorphous parts which becomes a trigger of the destruction as well as fluctuating the fibril structure to soften a whole fiber. It is preferable that the peak half-value width at Tm1 is 20° C. or higher so that the greater width makes the higher abrasion resistance. The upper limit of peak half-value width may industrially be around 80° C. and may be sufficient at 50° C.

Although only one endothermic peak is ordinarily observed in the liquid crystal polyester fiber, there may be a case of observing two or more endothermic peaks, when the fiber structure has been insufficiently solid-phase polymerized. In such a case, the peak half-value width is determined as the sum of the half-value widths of respective peaks.

It is preferable that the melting point (Tm1) of fiber is 290° C. or more, preferably 300° C. or more, and further preferably 310° C. or more. Such a high melting point makes the heat resistance of fiber excellent. To achieve such a high melting point of fiber, it is possible that a fiber is made from liquid crystal polyester having a high melting point. It is preferable that a melt-spun fiber is polymerized in a solid phase so that the fiber has a high strength and elastic modulus as well as excellent uniformity in a longitudinal direction. The upper limit of melting point may be around 400° C.

It is preferable that the heat of melting ΔHm1 is 6.0 J/g or less, although it varies depending upon the structural unit composition of the liquid crystal polyester. The ΔHm1 of 6.0 J/g or less can decrease the crystallinity, fluctuates the fibril structure and softens the fiber as a whole, and decreases the difference in structure between the crystal/amorphous parts which becomes a trigger of the destruction, so that the abrasion resistance improves. It is preferable that the ΔHm1 is 5.0 J/g or less so that the abrasion resistance improves. It is preferable that the ΔHm1 is 0.2 J/g or more, for achieving high strength and elastic modulus.

It is surprising that the ΔHm1 is 6.0 J/g or less in spite of high molecular weight of 250,000 or more. The liquid crystal polyester having a molecular weight of 250,000 or more is not fluidized with a remarkably high viscosity and is difficult to be melt-spun even above the melting point. A liquid crystal polyester fiber with such a high molecular weight can be obtained by melt spinning liquid crystal polyester having a low molecular weight to be subject to solid-phase polymerization. When the liquid crystal polyester fiber is subject to solid-phase polymerization, the molecular weight increases, the strength, elongation, elastic modulus and heat resistance improve, and the crystallinity also increases, so that the ΔHm1 increases. When the crystallinity increases, the strength, elongation, elastic modulus and heat resistance further increase, although the difference in structure between the crystal part and the amorphous part becomes remarkable, the interface therebetween is liable to be destroyed, and the abrasion resistance decreases. However in our invention, the high strength, elastic modulus and heat resistance can be maintained by having such a high molecular weight as characterized in a solid-phase polymerized fiber while the abrasion resistance can be increased by having such a low crystallinity or such a low ΔHm1 as observed in liquid crystal polyester without solid-phase polymerization. Our invention has achieved a technical advance improving the abrasion resistance by a structure change such as decreased crystallinity.

It is preferable that the Tm2 of the fiber is 300° C. or more from a viewpoint of enhanced heat resistance. The upper limit of Tm2 may be around 400° C.

It is preferable that the ΔHm2 is 5.0 J/g or less, preferably 2.0 J/g or less, because the excessive ΔHm2 might increase the crystallinity as a polymer itself and make it difficult to improve the abrasion resistance. Although only one endothermic peak is ordinarily observed in the liquid crystal polyester fiber when it is heated again after a cooling process in the above-described measurement condition, there may be a case of observing two or more endothermic peaks. In such a case, the ΔHm2 is determined as the sum of ΔHm2 of respective peaks.

The fiber has a peak temperature of loss tangent (tan δ) of 100° C. to 200° C., preferably 120° C. to 180° C. while it has a peak value of 0.060 to 0.090. In the specification, the peak temperature of tan δ and peak value are determined by the method to be described in Examples.

The tan δ is a ratio of loss elastic modulus to storage elastic modulus. When the tan δ is high the ratio of heat scatter per energy applied is high. It is thought that a peak appears in temperature dependence of tan δ in a synthetic fiber, and the peak temperature has significance like the glass transition temperature as a temperature at which kineticism of amorphous part begins to increase while the peak value has significance like the amount of the amorphous part itself.

The liquid crystal polyester fiber has a low crystallinity since it has been heat-treated at a high temperature after solid-phase polymerization, so that it consists primarily of the amorphous part and has a clear peak in the tan δ. The peak value corresponds to the amount of amorphous part and therefore the one having a high peak value has a great amount of amorphous part and tends to deform thermally. Namely, to suppress the thermal deformation, it is preferable that the peak temperature of tan δ is high and the peak value is low. On the other hand, to achieve a high abrasion resistance characterizing the fiber of our invention, it is preferable that the peak value is high so that the crystallinity of polymer is low. To achieve such conflicting characteristics at the same time, it is necessary to set the tan δ properly.

The tan δ peak value of the fiber should be 0.090 or less. The peak value of 0.090 or less can suppress thermal deformation at a high temperature. It is preferable that the peak value is 0.085 or less so that the thermal deformation is suppressed more. To prevent the abrasion resistance from deteriorating by a high crystallinity derived from an excessively low peak value, it is preferable that the peak value is 0.060 or more, preferably 0.065 or more.

The peak temperature of tan δ is a temperature at which the kineticism of amorphous part suddenly increases. The temperature above the peak temperature might cause a thermal deformation. Therefore, the peak temperature is preferably higher. The peak temperature of the fiber should be 100° C. or more, preferably 130° C. or more. The upper limit of peak temperature may be around 200° C.

As described later, such desirable peak temperature and peak value of tan δ can effectively be achieved by properly setting a stretch rate in a heat treatment after solid-phase polymerization.

To enhance the strength of mesh fabric, it is preferable that the liquid crystal polyester fiber has a strength of 12.0 cN/dtex or more, preferably 14.0 cN/dtex or more, further preferably 15.0 cN/dtex or more. The upper limit of strength may be around 30.0 cN/dtex.

It is preferable that the fiber has a strength fluctuation rate of 10% or less, preferably 5% or less. The strength in the specification means strength at a cutting process in measuring a tensile strength described in JIS L1013:2010. The strength fluctuation rate is measured by the method to be described in Examples. The uniformity along a longitudinal direction is enhanced and the fluctuation of fiber strength (product of strength and fineness) is decreased by the strength fluctuation rate of 10% or less, so that defects of fiber product reduce and yarn breakage derived from a low strength portion in a higher processing can also be suppressed.

To enhance the elastic modulus of fabric, it is preferable that the elastic modulus of fiber is 500 cN/dtex or more, preferably 600 cN/dtex or more, further preferably 700 cN/dtex or more. The upper limit of elastic modulus may be around 1200 cN/dtex.

It is preferable that the fiber has an elongation of 1.0% or more, preferably 2.0% or more. The elongation of 1.0% or more can enhance the impact absorbency of fiber to improve the abrasion resistance, and can make the processability in a higher processing and handling ability excellent. The upper limit of elongation may be around 10.0%. The fiber having a molecular weight of 250,000 or more can have a high elongation.

In the specification, strength, elongation and elastic modulus are determined by the method to be described in Examples.

Because of its high strength and elastic modulus, the fiber can be suitably used in applications, such as printing screen gauzes and meshes for filter. Also, because a high strength can be exhibited even with thin fiber fineness, it can be achieved to make a fibrous material smaller in weight and thickness, and a yarn breakage in a higher processing such as weaving process can also be suppressed. The fiber having a molecular weight of 250,000 or more can have a high strength and elastic modulus.

It is preferable that the fiber has a single fiber fineness of 18.0 dtex or less. Such a thin single fiber fineness of 18.0 dtex or less, can make the molecular weight easily increase to improve in strength, elongation and elastic modulus when polymerized in a solid phase at fibrous state. Further, it makes possible that the flexibility and the workability of fiber are improved, that the surface area increases to enhance the adhesion property with chemical agents such as an adhesive. Furthermore, it makes possible that the thickness becomes thinner, that the weave density is increased, and that the opening (area of opening part) can be widened in case of being formed as a gauze comprising monofilaments. The single-fiber fineness is more preferably 15.0 dtex or less, and further preferably 10.0 dtex or less. The lower limit of single fiber fineness may be around 1.0 dtex.

It is preferable that the fiber has a birefringent rate (Δn) of 0.250 or more and 0.450 or less. Such a range of the Δn can make the fiber axial molecular orientation sufficiently high to achieve high strength and elastic modulus.

It is preferable that the fiber has an abrasion resistance C of 60 sec or more, preferably 90 sec or more, further preferably 180 sec or more. The abrasion resistance C is determined by the method to be described in Examples. The abrasion resistance C of 60 sec or more can make it possible that fibrillation of liquid crystal polyester fiber at a higher processing is suppressed, that deterioration of the processability and weaving performance causes by fibril accumulation is suppressed, that the clogging of opening due to accumulated fibrils being woven therein is suppressed, and that less deposition of fibrils onto guides extends the cycle for cleaning or exchange.

It is preferable that the fiber has a thermal deformation rate at a high temperature of 1.0% or less. The thermal deformation rate of 1.0% or less can maintain a product performance even after a high-temperature heat treatment. It is preferable that the thermal deformation rate is 0.7% or less. The lower limit of thermal deformation rate may be around 0.2%.

To make fiber products thinner and lighter, it is preferable that the fiber has the number of filaments of 50 or less, preferably 20 or less. In particular, such a fiber can be suitably used in the technical field of monofilament having the number of filaments of 1 requiring high fiber fineness, high strength, high elastic modulus and high uniformity of single fiber fineness.

It is preferable that the fiber has a yarn length of 40,000 m or more. The length of 40,000 m can minimize faults caused by connecting yarns in product-making process such as weaving process. The upper limit of yarn length may be around 10,000,000 m although the longer is the more preferable. Such a long yarn length of fiber can effectively be prepared under conditions of a proper stretch rate and a good running stability achieved by regulating a yarn route with a guide after heat treatment.

A mesh fabric can be made from the liquid crystal polyester fiber. Since the liquid crystal polyester fiber is excellent in abrasion resistance and processability, the weaving performance in making a product such as a mesh fabric is enhanced to make the product with less faults. Further, the thermal deformation is small even at a high temperature, so that the product doesn't change greatly in dimension and performance even in a high-temperature processing.

The liquid crystal polyester fiber has a high strength, high elastic modulus and high abrasion resistance and a small thermal deformation, and is excellent in processability, so that it can be used in various fields such as general industrial material, civil engineering and construction material, sport material, protective clothing material, rubber-reinforcing material, electric material (tension members in particular), acoustic material and general clothing material. It can suitably be used for screen gauzes, filters, ropes, nets, fishing nets, computer ribbons, base fabrics for printed boards, canvases for paper machines, air bags, air ships, base fabrics for domes or the like, rider suits, fishlines, various lines (lines for yachts, paragliders, balloons, kite yarns or the like), blind cords, support cords for wire screens, various cords in automobiles or air planes, power transmission cords for electric equipment or robots or the like. It can be particularly suitable as woven fabrics for industrial materials comprising monofilaments such as preferably used for printing screen gauzes and filters, for such monofilaments which strongly require high strength, high elastic modulus and thin fineness as well as good abrasion resistance for improving weaving performance and fabric quality.

Hereinafter a method for producing the liquid crystal polyester fiber will be explained.

The composition and desirable composition ratio of the liquid crystal polyester have been described in the part explaining fibers.

To make a wider temperature range capable of melt spinning, it is preferable that a melting point of the liquid crystal polyester is 200 to 380° C., and is preferably 250 to 360° C. for enhancing spinnability. The melting point of the liquid crystal polyester polymer means a value (Tm2) measured by the method to be described in Examples.

It is preferable that the liquid crystal has a weight average molecular weight (may be called “molecular weight”) of 30,000 or more in terms of polystyrene. The molecular weight of 30,000 or more can enhance the yarn-making property with an adequate viscosity at a spinning temperature. When the molecular weight is too high, the viscosity becomes high and the flowability deteriorates although the strength, elongation and elastic modulus of the fiber are enhanced, and ultimately it becomes impossible to flow. Therefore it is preferable that the molecular weight is 250,000 or less, preferably less than 200,000 or less. The weight average molecular weight in terms of polystyrene is determined by the method to be described in Examples.

It is preferable that the liquid crystal polyester is dried before being melt spun, from a viewpoint of suppressing bubbling caused by water mixture and of enhancing yarn-making property. It is more preferable that vacuum drying is performed, because the monomer which remains in the liquid crystal polyester can be removed, so that yarn-making property is further enhanced. The vacuum drying is usually performed at 100-200° C. for 8-24 hours.

To prevent a systematic structure from being produced at the time of polymerization in the melt spinning, it is preferable to use an extruder-type extruding machine although any known method can be employed for melt extrusion of liquid crystal polyester. The extruded polymer is metered by a known metering device, such as a gear pump through a pipe, and is introduced into a spinneret after passing through a filter for removing foreign materials. It is preferable that the temperature (spinning temperature) from the polymer pipe to the spinneret is controlled above the melting point of the liquid crystal polyester, preferably controlled to a temperature of the melting point of the liquid crystal polyester+10° C. or more. It is preferable that the spinning temperature is 500° C. or less, preferably 400° C. or less, in case that the spinning temperature is so high that the viscosity of the liquid crystal polyester increases to deteriorate fluidity and yarn-making property. It is possible to individually adjust the temperature at each portion from the polymer pipe to the spinneret. In this case, the discharge can be stabilized by controlling the temperature of a portion near the spinneret as higher than the temperature of an upstream portion thereof.

To enhance the yarn-making property and uniformity of fineness with the discharge, it is preferable that the spinneret has a hole of small diameter and a long land length (length of a straight pipe part having the same inner diameter as the hole of the spinneret). It is preferable that the hole diameter is 0.05 mm or more and 0.50 mm or less, preferably 0.10 mm or more and 0.30 mm or less, in case that an excessively small hole diameter might cause a clogging of holes. It is preferable that an L/D defined as a quotient calculated by dividing land length L with hole diameter D is 1.0 or more and 3.0 or less, preferably 2.0 or more and 2.5 or less, in case that an excessively long land length might increase a pressure loss.

To maintain the uniformity, it is preferable that the spinneret has holes of 50 or less, preferably 20 or less. It is preferable that an introduction hole positioned right above the hole of the spinneret is straight shaped hole, from a viewpoint of preventing the increased pressure loss. It is preferable that the introduction hole and the spinneret hole are connected with a tapered portion to suppress abnormal retention.

The polymer discharged from the spinneret holes passes through heat retention region and cooling region and is solidified and then is drawn up by a roller (godet roller) rotating at a constant speed. It is preferable that the heat retention region extends by a length of 200 mm or less from the spinneret surface, preferably 100 mm or less, because the yarn-making property deteriorates by an excessively long heat retention region. When the atmosphere temperature in the heat retention region is raised with a heating means, it is preferable that the atmosphere temperature is 100° C. or more and 500° C. or less, preferably 200° C. or more and 400° C. or less. The polymer can be cooled with inert gas, air, steam or the like. To reduce the environmental load and energy, it is preferable that it is cooled with air flow at room temperature (20-30° C.) blown in parallel or annularly.

From viewpoints of improved productivity and thinner single-yarn fineness, it is preferable that the draw velocity (spinning velocity) is 50 m/min or more, preferably 500 m/min or more. Since the desirable liquid crystal polyester has a good spinnability at a spinning temperature, the upper limit of draw velocity may be around 2,000 m/min.

It is preferable that a spinning draft defined as a quotient calculated by dividing a draw velocity with a discharge linear velocity is 1 or more and 500 or less, and is more preferably 10 or more and 100 or less to enhance a yarn-making property and uniformity of fineness.

In a melt spinning process, it is preferable that oil solution is applied between a cooling-solidification step of polymer and a take-up step so that the handling property of fiber is improved. The oil solution may be a known oil solution and is preferably a general spinning oil solution or a mixed oil solution of inorganic particle (A) and phosphate compound (B) to be described later, in order to improve an unraveling-property to unravel a fiber (hereinafter called raw yarn of spinning) prepared by melt-spinning at a roll-back step before solid-phase polymerization.

The take-up may be carried out by using a known winder to form a package such as pirn, cheese and cone. To prevent a fiber from fibrillating with friction, it is preferable to employ a pirn winding in which a roller doesn't contact a package surface when the fiber is taken up.

It is preferable that the melt-spun fiber has a single fiber fineness of 18.0 dtex or less. The single fiber fineness is determined by the method to be described in Examples. The single fiber fineness of 18.0 dtex or less can increase the molecular weight of polymer constituting the fiber at the time of solid-phase polymerization in a fiber state, so that strength, elongation and elastic modulus are improved. Further, the surface area can be wider to increase the adhesion amount of fusion inhibitor of inorganic particle (A) and phosphate compound (B). It is preferable that the single fiber fineness is 10.0 dtex or less, preferably 7.0 dtex or less. The lower limit of single fiber fineness may be around 1.0 dtex.

It is preferable that the melt-spun fiber has a strength of 3.0 cN/dtex or more, preferably 5.0 cN/dtex or more so that the processability is enhanced by preventing yarn breakage in a roll-back process before the solid-phase polymerization. The upper limit of strength may be around 10 cN/dtex.

It is preferable that the melt-spun fiber has an elongation of 0.5% or more, preferably 1.0% or more so that the processability is enhanced by preventing yarn breakage in a roll-back process before the solid-phase polymerization. The upper limit of elongation may be around 5.0%.

It is preferable that the melt-spun fiber has an elastic modulus of 300 cN/dtex or more, preferably 500 cN/dtex or more so that the processability is enhanced by preventing yarn breakage in a roll-back process before the solid-phase polymerization. The upper limit of elastic modulus may be around 800 cN/dtex.

The strength, elongation and elastic modulus are determined by the method to be described in Examples.

It is preferable that the melt-spun fiber has a molecular weight of 30,000 or more. The molecular weight of 30,000 or more can achieve a high strength, elongation and elastic modulus with excellent processability. It is preferable that the molecular weight is 250,000 or less, preferably 200,000 or less, because excessively high molecular weight might slow the solid-phase polymerization to fail to have a high molecular weight achieved. The weight average molecular weight in terms of polystyrene is determined by the method to be described in Examples. Besides, the molecular weight doesn't tend to fluctuate greatly in a melt spinning process.

Then the melt spun fiber is subject to solid-phase polymerization after fusion inhibitor oil solution is applied to the fiber. To enhance the adhesion efficiency, it is preferable that the fusion inhibitor is applied to the fiber yarn while a melt spun fiber yarn taken up is rolled back, or that the fusion inhibitor is applied in a small amount to the melt spun fiber yarn and then is applied additionally to the fiber while the taken-up fiber yarn is rolled back, although the fusion inhibitor may be applied to the fiber between the melt spinning and take-up processes.

To make the fusion inhibitor uniformly adhere to a fiber such as monofilament having a thin total fineness, it is preferable that the fusion inhibitor is applied with a kiss roll (oiling roll) made of metal or ceramic, although a guide-feed method may be employed for the adhesion. A hank or a tow of fiber can be applied by immersing it in a mixed oil solution.

It is preferable that the fusion inhibitor is a mixture of inorganic particle (A) and phosphate compound (B). The mixture of inorganic particle (A) and phosphate compound (B) applied can suppress the fusion between fibers in solid-phase polymerization and thermally denature the components in the solid-phase polymerization process, to achieve excellent processability in the following process and excellent post-workability to make a product. In the specification, the fusion inhibitor made of inorganic particle (A) and phosphate compound (B) is called “oil solution for solid-phase polymerization”, “mixed oil solution” or “oil solution” for convenience although such an oil solution doesn't contain any oil component.

The inorganic particle (A) in the specification is a known inorganic particle and may be mineral, metal hydroxide such as magnesium hydroxide, metal oxide such as silica and alumina, carbonate compound such as calcium carbonate and barium carbonate, sulfate compound such as calcium sulfate or barium sulfate, carbon black, or the like. Such a heat-resistant inorganic particle is applied onto the fiber to reduce contact areas between single fibers in solid-phase polymerization, so that fusion is prevented in the solid-phase polymerization process.

It is preferable that the inorganic particle (A) is easily handled to perform the application process while it is easily dispersed in water to reduce environmental load and is inert under a solid-phase polymerization condition. From these viewpoints, it is preferable to employ silica or mineral of silicate. It is preferable that the mineral of silicate is a phyllo-silicate having a layer structure. The phyllo-silicate may be kaolinite, halloysite, serpentine, garnierite, smectites, pyrophyllite, talc, mica or the like. From a viewpoint of availability, it is most preferable to employ talc or mica.

The phosphate compound (B) may be a compound identified by any one of following chemical formulae (1)-(3).

Here, R1 and R2 indicate hydrocarbon, M1 indicates alkali metal, M2 indicates any one of alkali metal, hydrogen, hydrocarbon and oxygen-containing hydrocarbon. Besides n indicates an integer of 1 or more. From a viewpoint of suppressing thermolysis, it is preferable that the upper limit of n is 100 or less, preferably 10 or less.

From a viewpoint of reducing the environmental load of gas generated with thermolysis in solid-phase polymerization, it is preferable that the R1 has no phenyl group in the structure and preferably consists of alkyl group. From a viewpoint of affinity to the fiber surface, it is preferable that the R1 has a carbon number of 2 or more. From a viewpoint of suppressing the weight reduction rate caused by decomposition of organic components accompanied with solid-phase polymerization to prevent carbide generated by the decomposition in the solid-phase polymerization process from remaining on the fiber surface, it is preferable that the carbon number is 20 or less.

From a viewpoint of water solubility, it is preferable that the R2 is a hydrocarbon having a carbon number of 5 or less, preferably 2 or 3.

From a viewpoint of production cost, it is preferable that the M1 is sodium or potassium.

Using both inorganic particle (A) and phosphate compound (B) can enhance the dispersibility of inorganic particle (A) and enable uniform application to fiber to exhibit excellent suppression of fusion and adhesion of inorganic particle (B) onto the fiber surface, so that decreased amount of inorganic particle (A) remains on the fiber after a washing process and then fouling is suppressed in the following processing.

Further, phosphate compound (B) can easily be removed with water from fiber in the washing process after solid-phase polymerization, through generating condensed phosphate salt with dehydration and decomposition of organic components contained in phosphate compound (B) under a solid-phase polymerization condition. When phosphate compound (B) is solely applied to fiber, the deliquescence of the condensed salt might make the phosphate salt absorb moisture to deliquesce on the fiber surface even under an ordinary fiber storage condition, so that washability deteriorates because of increased viscosity. Namely, the excellent washability is exhibited by using both inorganic particle (A) and phosphate compound (B). We presume such an excellent washability is exhibited by a mechanism in which inorganic particle (A) having a good absorbency prevents the condensed salt of phosphate compound (B) from naturally absorbing moisture to deliquesce and the condensed salt of phosphate compound (B) absorbs water to expand as running in water, so as to fall off the fiber surface by layer fractions.

To uniformly apply inorganic particle (A) and phosphate compound (B) to fiber by an adequate adhesion amount, it is preferable to employ a mixed oil solution made by adding inorganic particle (A) to diluted solution of phosphate compound (B) which is preferably diluted with water for safety. From a viewpoint of suppressing fusion, it is preferable that the concentration of inorganic particle (A) is as high as 0.01 wt % or more, preferably 0.1 wt % or more and that the upper limit is 10 wt % or less, preferably 5 wt % or less for uniform dispersion. From a viewpoint of uniform dispersion, it is preferable that the concentration of phosphate compound (B) is as high as 0.1 wt % or more, preferably 1.0 wt % or more. To prevent the mixed oil solution from excessive adhesion caused by increased viscosity and adhesive spotting caused by temperature dependency of viscosity, it is preferable that the concentration of phosphate compound (B) is 50 wt % or less, preferably 30 wt % or less.

It is preferable that “a” defined as adhesion rate of inorganic particle (A) and “b” defined as adhesion rate of phosphate compound (B) satisfy the following conditions.

30≧a+b≧2.0  Condition 1:

a≧0.05  Condition 2:

b/a≧1  Condition 3:

In Condition 1, it is preferable that the oil adhesion rate (a+b) of oil solution for solid-phase polymerization is 2.0 wt % or more for suppressing fusion, and is 30 wt % or lower in case that excessive adhesion rate might make fiber sticky to deteriorate the handling ability. It is more preferably 4.0 wt % or more and 20 wt % or less. Here, the oil adhesion rate (a+b) of oil solution for solid-phase polymerization is determined by the method to be described in Examples for fiber after applying the oil solution for solid-phase polymerization.

In Condition 2, the adhesion rate (a) of inorganic particle of 0.05 wt % or more can suppress fusion by inorganic particles remarkably. The upper limit of adhesion rate (a) may be around 5 wt % or less, from a viewpoint of uniform adhesion.

In Condition 3, it is preferable that adhesion rate (b) of phosphate compound (B) is equal to or more than adhesion rate (a) of inorganic particle (A), so that the adhesion between inorganic particle (A) and fiber is suppressed while excellent washability is exhibited remarkably as derived from generating condensed salt in solid-phase polymerization of phosphate compound (B).

Here, adhesion rate (a) of inorganic particle (A) and adhesion rate (b) of phosphate compound (B) are calculated by the following formula.

(Adhesion rate (a) of inorganic particle (A))=(a+b)×Ca/(Ca+Cb)

(Adhesion rate (b) of phosphate compound (B))=(a+b)×Cb/(Ca+Cb)

Here, Ca indicates a concentration of inorganic particle (A) in oil solution for solid-phase polymerization, Cb indicates a concentration of phosphate compound (B) in oil solution for solid-phase polymerization.

Next, the melt spun liquid crystal polyester fiber is subject to solid-phase polymerization. The solid-phase polymerization can increase the molecular weight to increase strength, elastic modulus and elongation. The solid-phase polymerization may be performed to a hank or tow of fiber (placed on a metal net or the like) or a continuous yarn between rollers. To simplify the apparatus and improve the productivity, it is preferable to be performed to a package made by taking up the fiber on a core.

When the solid-phase polymerization is performed to the package, the winding density of fiber package in solid-phase polymerization should be important to prevent the fusion prevention. To prevent a winding collapse, it is preferable that the winding density is 0.01 g/cc or more. It is preferable that the winding density is 1.0 g/cc or less, preferably 0.8 g/cc or less to prevent the fusion-bonding. Here, the winding density is calculated from fiber weight Wf [g] and occupied volume Vf [cc] of package obtained from outer size of package and core bobbin size. In case of package collapse by excessively small winding density, it is preferable that the winding density is 0.1 g/cc or more. The occupied volume Vf is determined by actually measuring the outer size of package or by calculating from the outer size measured on picture as assuming that the package is rotationally symmetric. The Wf is determined by actually measuring the weight difference before and after winding or by calculating from fineness and winding length.

It is preferable to form such a package having a small winding density when the package has been taken up in melt spinning because the productivity for apparatus and the efficiency of production can be improved. On the other hand, it is preferable to make the winding density small when the package has been taken up in melt spinning and then rolled back because the winding tension can be small for the smaller winding density. Because the winding density can be smaller by the smaller winding tension in the roll-back, it is preferable that the winding tension is 0.50 cN/dtex or less, preferably 0.30 cN/dtex or less. The lower limit of winding density may be around 0.01 cN/dtex.

To decrease the winding density, it is preferable that the roll-back velocity is 500 m/m or less, preferably 400 m/m or less. On the other hand, a higher roll-back velocity is advantageous for productivity and it is preferable that the roll-back velocity is 50 m/m or more, preferably 100 m/m or more.

In order to form a stable package even with a low tension, it is preferable that the winding formation is a taper-end winding provided with tapered both ends. It is preferable that the taper angle is 70° or less, preferably 60° or less. When long fiber is required and the taper angle is too small to make a large fiber package, it is preferable that the taper angle is 1° or more, preferably 5° or more. In the specification, the taper angle is defined by the following formula.

$\begin{matrix} {\theta = {\tan^{- 1}\left( \frac{2d}{l_{i} - l_{o}} \right)}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \end{matrix}$

θ: taper angle [°], d: winding thickness [mm], innermost stroke [mm], lo: outermost stroke [mm]

The winding number is also important for forming a package. The winding number means the number of times of rotation of a spindle during half reciprocation of a traverse. It is defined as a product of a time for the half reciprocation of a traverse [min] and the rotational speed of a spindle [rpm]. The greater winding number indicates the smaller traverse angle. A smaller winding number is advantageous for avoiding fusion-bonding because the contact area between fibers becomes smaller while a greater winding number makes a good shape of package by reducing the package expansion and traverse failures at end faces. From these viewpoints, it is preferable that the winding number is 2 or more and 20 or less, preferably 5 or more and 15 or less.

The bobbin used for forming the fiber package may be any type bobbin as long as it has a cylindrical shape, and it is attached to a winder when taken up, and fiber is taken up to form a package by rotating it. In solid-phase polymerization, although the fiber package may be treated integrally with the bobbin, the treatment may be carried out in a condition where only the bobbin is taken out from the fiber package. When the treatment is carried out in a condition where fiber is wound on the bobbin, the bobbin should resist the temperature of solid-phase polymerization and is preferably made of metal such as aluminum, brass, iron and stainless steel. It is preferable that many holes are opened on the bobbin so that by-product of polymerization is removed quickly to perform solid-phase polymerization efficiently. When the treatment is carried out in a condition where the bobbin is taken out from the fiber package, it is preferable that an outer skin is attached onto the outer layer of bobbin. To prevent fusion between fiber in the innermost layer of package and bobbin outer layer in both cases, it is preferable that cushion material is wound around the outer layer of bobbin onto which liquid crystal polyester melt-spun fiber is taken up. It is preferable that the cushion material is made of felt comprising organic fiber or metal fiber, and has a thickness of 0.1 mm or more and 20 mm or less. The above-described outer skin may be replaced by the cushion material.

It is preferable that the fiber package has a yarn length (winding amount) of 10,000 m or more and 10,000,000 m or less.

The solid-phase polymerization may be performed under atmosphere of inert gas such as nitrogen or atmosphere of active gas, such as air, containing oxygen, or under reduced pressure condition. To simplify the apparatus and prevent fiber or core material from oxidizing, it is preferable that it is performed under nitrogen atmosphere. It is preferable that the solid-phase polymerization is performed under atmosphere of low-humidity gas having a dew point of −40° C. or lower.

It is preferable that the maximum temperature of solid-phase polymerization is Tm1−60° C., where Tm1 [° C.] is defined as an endothermic peak temperature of the liquid crystal polyester fiber to be subject to solid-phase polymerization. Such a high temperature around the melting point makes it possible for the solid-phase polymerization to progress immediately, so as to improve the fiber strength. The Tm1 means a melting point of liquid crystal polyester fiber and is determined by the measurement method to be described in Examples. To prevent fusion-bonding, it is preferable that the maximum temperature is less than Tm1 [° C.]. It is preferable that the solid-phase polymerization temperature is increased stepwise or continuously to time, to prevent fusion-bonding and improve time efficiency of solid-phase polymerization. In this case, because the melting point of the liquid crystal polyester fiber increases together with progress of solid-phase polymerization, the solid-phase polymerization temperature can be raised up to Tm1+100° C. of the liquid crystal polyester fiber before solid-phase polymerization process. In this case, it is preferable that the maximum temperature during solid-phase polymerization is Tm1−60 [° C.] or more and less than Tm1 [° C.] of the fiber after solid-phase polymerization, so that the solid-phase polymerization speed is increased and fusion-bonding is prevented.

To sufficiently enhance the molecular weight or strength, elastic modulus and elongation of fiber, it is preferable that the solid-phase polymerization time is 5 hours or more, preferably 10 hours or more. On the other hand, it is preferable that the time is 100 hours or less, preferably 50 hours or less to improve productivity because effects of enhanced strength, elastic modulus and elongation are saturated over time.

From viewpoints of processability in the higher processing and suppressed faults in appearance of product, it is preferable that solid-phase polymerized fiber is washed. The fiber is washed to remove oil solution for solid-phase polymerization to prevent fusion-bonding, so that processability deterioration, which might be caused by depositing the oil solution for solid-phase polymerization on guides in a post process such as weaving process, and fault generation, which might be caused by contaminating depositions in products, are suppressed.

The washing method may be a method of wiping the fiber surface with cloth or paper. In case that the solid-phase polymerized yarn might fibrillate with kinetic load, it is preferable to immerse the fiber in a liquid to which the oil solution for solid-phase polymerization is soluble or dispersible. It is more preferable that the washing is performed by blowing off with fluid in addition to the immersing in liquid, so that the oil solution for solid-phase polymerization expanded with liquid is removed efficiently.

It is preferable that the washing liquid is water for reducing environmental load. The liquid temperature should be higher for enhancing removal efficiency and is preferably 30° C. or more, preferably 40° C. or more. Because the liquid might evaporate remarkably when the liquid temperature is too high, it is preferable that the liquid temperature is the liquid boiling point −20° C. or less, preferably the liquid boiling point −30° C. or less.

From a viewpoint of washing efficiency improvement, it is preferable that a surfactant is added to the washing liquid. To increase the removal rate and decrease the environmental load, it is preferable that a surfactant is added by 0.01-1 wt %, preferably 0.1-0.5 wt %.

It is preferable that vibration or liquid flow is applied to a liquid for washing to enhance washing efficiency. From viewpoints of simplifying the apparatus and saving energy, it is preferable that the liquid flow is applied to the liquid, although ultrasonic vibration may be applied to the liquid. The liquid flow may be applied with a nozzle or by stirring in a liquid bath. It is preferable that it is applied with a nozzle so that the liquid is easily circulated with the nozzle through the liquid bath.

To increase the washing load per hour, it is possible that a hank, tow or package of fiber is immersed in the liquid. It is preferable that the fiber running continuously is immersed in the liquid. The method to immerse the fiber continuously may be performed by leading the fiber with a guide or the like into the liquid bath. To suppress fibrillation of solid-phase polymerization caused by contact resistance to the guide, it is preferable that both ends are provided with a slit through which fiber flows in the bath without yarn route guide.

Fiber is unraveled from a package of solid-phase polymerized yarn continuously fed. To suppress fibrillation in delamination of slight fusion-bonding caused by solid-phase polymerization, it is preferable that the yarn is unraveled in a direction (fiber-rounding direction) perpendicular to rotation axis by lateral-unraveling while the solid-phase polymerized package is rotated.

Such an unraveling may be performed by a method such as forcing the yarn to be driven at a constant rotation speed by a motor or the like, controlling the rotation speed with a dancer roller to regulate the unraveling speed, and drawing the yarn from the solid-phase polymerized package placed on a free roll with a speed-regulating roller to perform the unraveling. To remove oil efficiently, it is preferable that a package of liquid crystal polyester fiber is immersed in the liquid and then is unraveled as is.

It is preferable that the fluid used to blow off is air or water. It is particularly preferable that the fluid is air to dry the surface of liquid polyester fiber to improve yield by preventing contaminant deposition in a post-processing.

Next, the solid-phase polymerized fiber is heat-treated at a temperature of the melting point+50° C. or more. The melting point is Tm1 determined by the method to be described in Examples. Hereinafter, the melting point of fiber may be called Tm1. The abrasion resistance greatly improves when liquid crystal polyester fiber is heat treated at a temperature as high as Tm1+50° C. or more. The effect will become remarkable when the single fiber fineness is small.

A rigid molecular chain like liquid crystal polyester has a long relaxation time and inner layer also relaxes within the relaxation time for surface layer as melting the fiber. By studying technologies suitable for liquid crystal polyester fiber to improve abrasion resistance, it was found that abrasion resistance of liquid crystal polyester fiber can be improved by heating to reduce crystallinity and crystal completeness as a whole fiber instead of relaxation of molecular chain.

To reduce crystallinity, fiber has to be heated above the melting point. However a thermoplastic synthetic fiber might reduce strength and elastic modulus and cause thermal deformation and fusion (meltdown) at such a high temperature particularly in case of small single-fiber fineness. Such a behavior was seen with liquid crystal polyester, however, we focused on the melting point of liquid crystal polyester as a temperature transiting from crystal to liquid crystal and found out that increase of molecular weight of solid-phase polymerized liquid crystal polyester has made relaxation time very long so that the molecular mobility of liquid crystal is low. Therefore even with a short-time heat treatment at a high temperature above the melting point, the crystallinity can be reduced as keeping the orientation of molecular chains at a high level while the strength and the elastic modulus are not greatly deteriorated. From these facts, it was found that liquid crystal polyester fiber having a small single-yarn fineness can be improved in abrasion resistance by a short-time heat treatment at a high temperature above Tm1+50° C. without great loss of strength, elastic modulus and heat resistance of liquid crystal polyester fiber.

To lower the crystal completeness for the solid-phase polymerized fiber, it is preferable that the heat treatment is performed at a temperature of Tm1+60° C. or more, preferably Tm1+80° C. or more, most preferably Tm1+130° C. or more. In case that excessively high treatment temperature might increase the heat deformation of processed fiber at a high temperature, it is preferable that the heat treatment is performed at a temperature of Tm1+200° C. or less, preferably Tm1+180° C. or less.

Although there is a case for carrying out a heat treatment for liquid crystal polyester fiber even in a conventional technology, it is generally carried out at a temperature of the melting point or less because the liquid crystal polyester is thermally deformed (fluidized) by stress even at a temperature of the melting point or less. Even when the solid-phase polymerization of liquid crystal polyester fiber is performed as a heat treatment, the treatment temperature should be set below the melting point of fiber or the fiber might be fused and melt down. In case of solid-phase polymerization, the final temperature of solid-phase polymerization may increase to a temperature higher than the melting point of fiber to be treated because the melting point of fiber may increase through the treatment. Even in this case, the treatment temperature is lower than the melting point of fiber being treated, that is, the melting point of fiber after the heat treatment.

Such a high-temperature heat treatment, which doesn't mean the solid-phase polymerization, increases abrasion resistance by decreasing a structural difference between a dense crystal portion formed by solid-phase polymerization and an amorphous portion, namely by decreasing the crystallinity and crystal completeness. Therefore even if Tm1 is varied by heat treatment, it is preferable that the heat treatment is performed at a temperature of Tm1, which is varied after the treatment, +50° C. or more, preferably the Tm1+60° C. or more, further preferably the Tm1+80° C. or more, most preferably the Tm1+130° C. or more.

Although heat stretching of liquid crystal polyester fiber may be included in the heat treatment, the heat stretching is a process tensing the fiber at a high temperature, the orientation of molecular chain in the fiber structure becomes high, the strength and the elastic modulus increase, and the crystallinity and crystal completion are maintained as they are, namely, high AHm 1 is maintained and the small peak half-value width of the melting point is maintained. Therefore it becomes a fiber structure being inferior in abrasion resistance and such a heat stretching should be different from our heat treatment that aims to improve the abrasion resistance by decreasing the crystallinity (decreasing AHm 1) and decreasing the crystal completion (increasing the peak half-value width). In our high-temperature heat treatment, the crystallinity decreases so that strength and elastic modulus do not increase.

It is preferable that the high-temperature heat treatment is performed as running fiber continuously, because the fusion-bonding between fibers can be prevented and enhance the uniformity of the treatment. To prevent fibrils from generating as achieving uniform treatment, it is preferable that a non-contact heat treatment is performed. The heat treatment may be performed by heating the atmosphere or a radiation heating with a laser or an infrared ray or the like. It is preferable that it is performed with a slit heater having a block or a plate heater so that both advantages of atmosphere heating and radiation heating enhance the stability for the treatment.

The high-temperature heat treatment should be performed at a stretch rate of 0.1% or more and less than 3.0%. In the specification, the stretch rate is defined by the following formula with yarn velocity (V0) before heat treatment and yarn velocity (V1) after heat treatment. The yarn velocities before and after heat treatment have the same meaning as the surface velocities of roller regulating the yarn velocity before and after heat treatment.

(Stretch rate [%])=(V1−V0)×100/V0

The stretching and relaxing in a high-temperature heat treatment have been described in prior art documents although that only meant a high stretch could make fiber thinner in addition to improvement of running stability or abrasion resistance. However, it was found that stretching in a heat treatment contribute to suppression of thermal deformation particularly at a high temperature from a viewpoint of achieving both improved abrasion resistance and suppressed thermal deformation. We assume the reason is as follows.

The high-temperature heat treatment is carried out at a temperature as high as the melting point+50° C. or more as described above. At this temperature, crystal portions of liquid crystal polyester fiber melt to be amorphous (liquid crystal) with orientation. Prior arts have aimed to disturb the orientation of the amorphous material by heat relaxation at such a high temperature.

It seems that the solid-phase polymerized liquid crystal polyester fiber has a restriction point of which interaction is strong. Such a restriction point makes it difficult to sufficiently disturb the orientation of the amorphous material by heat relaxation only. If the heat-treatment temperature is increased to sufficiently disturb it, the heat relaxation is enhanced to disturb the orientation of the amorphous material greatly, so that thermal deformation becomes great at a high temperature. In other words, it is difficult only by adjustment of the heat-treatment temperature to achieve both the high abrasion resistance and suppression of thermal deformation at a high temperature.

Therefore proper stretch is important. When the liquid polyester in an amorphous (liquid crystal) state oriented under high-temperature heat treatment is deformed slightly in a longitudinal fiber axial direction, the restriction point is destroyed while the orientation relaxation is suppressed by flow deformation. That effect reduces interaction between liquid crystal polyester to adjust the disturbance of orientation within a proper range to achieve both the high abrasion resistance and suppression of thermal deformation.

According to our assumption described above, higher temperature and higher stretch rate could be effective. However, the higher stretch could contribute to destroying the restriction point greatly from 0% to 3% of stretch while the effect would be saturated above the range. On the other hand, to make the stretch rate higher, it is necessary to reduce resistance against elongation deformation, namely elongation viscosity, while it is necessary to increase heat-treatment temperature. In such a case, thermal deformation cannot be suppressed since the effect of the increased heat-treatment temperature surpasses the effect of stretch.

Our invention is characterized by an advantage that the improvement of abrasion resistance of liquid crystal polyester fiber, which has conventionally been controlled only by high-temperature heat-treatment temperature, can be controlled separately with interaction increase and orientation disturbance by a proper stretch. Such a characteristic achieve both the higher abrasion resistance and suppression of thermal deformation.

The stretch rate should be 0.1% or more. The stretch rate of 0.1% or more can achieve the improvement of abrasion resistance. To improve the abrasion resistance, it is preferable that the stretch rate is as high as 0.5% or more, preferably 0.6% or more. On the other hand, in case that excessively high stretch rate might have too much disturbance of orientation of amorphous material to increase thermal deformation at a high temperature, it is preferable that the stretch rate is less than 3.0%, preferably less than 2.5%.

It is preferable that the treatment velocity (yarn velocity) is 100 m/min or more, preferably 200 m/min or more, further preferably 300 m/min or more, so that the short-time processing can be achieved at a high temperature while the abrasion resistance and productivity are improved although depending on treatment length. The upper limit of processing velocity may be around 1,000 m/min from a viewpoint of running stability of fiber.

It is preferable that the treatment length (heater length) is 100 mm or more, preferably 500 mm or more, from a viewpoint of uniform processing in a case of non-contact heating although depending on heating method. It is preferable that it is 3,000 mm or less, preferably 2,000 mm or less, in case that too long treatment length might cause non-uniform processing and fiber meltdown by yarn sway inside a heater.

It is preferable that the fiber which has been heat treated at a high temperature is taken up under a yarn route regulation with yarn route guide in a range of 1 cm or more and 50 cm or less from the fiber heating region.

We found in a long-run evaluation that when a proper stretch is performed to slightly extend the fiber in heat treatment, fluctuation of stretch point might cause a longitudinal unevenness of fiber and yarn breakage. We assume that the stretch point fluctuates because the tension is small enough to cause the yarn sway in the heat treatment at a temperature as high as the melting point+50° C. or more. If the stretch rate were 0%, the fiber wouldn't be extended at all and a possible yarn sway wouldn't cause the yarn breakage. It seems that the stretch causes the effect of yarn sway.

Therefore the regulation using the guide to reduce yarn sway is effective. The liquid crystal polyester fiber before the high-temperature heat treatment can be fibrillated by scratch while the one after the heat treatment cannot be fibrillated by scratch at a low tension since it already has an abrasion resistance enhanced.

It is preferable that the yarn route guide is provided in a position range of 1 cm or more and 50 cm or less from the heating region. Since the fiber is cooled (air-cooled) after exiting the heating region, it deforms slightly as being cooled even after exiting the heating region. The effect of yarn sway is greatest in this region, and it is preferable that the position range is 1 cm or more and 50 cm or less as a cooling region, preferably 1 cm or more and 20 cm or less.

It is preferable that one or more guides are provided. It is preferable that three or less guides are provided because too many guides might increase frequency of scratch to increase the possibility of fibrillation. It is also preferable that a fiber is fed among a plurality of guides arranged in a fiber running direction. In this case the position of provision means a position of guides closest to the heater.

The guide may be made of general material such as ceramic and metal. To reduce damage to liquid crystal polyester fiber, it is preferable that it has a metal surface plated with hard chrome. To keep a proper coefficient of friction not to damage fiber, it is preferable that the surface roughness is 2 to 8, preferably 2 to 4 in terms of Rzjis determined by the method of JIS B0601:2001.

When the fiber contacts the guide, the running tension ratio before and after the guide should not be too high to reduce damage to fiber. It is preferable that a ratio of T2/T1 is 1.0 or more and 2.0 or less, where the running tension (T2) is a tension in a region closer to the winding side than the guide, and the running tension (T1) is a tension in a region closer to the heating region.

In the last, a fiber structural change in high-temperature heat treatment will be explained from a viewpoint of difference in fiber characteristics before and after processing.

Such a heat treatment means a short-time heat treatment at a high temperature no less than the melting point (crystal—liquid crystal transition temperature) of liquid polyester fiber, where the crystallinity decreases but the orientation slightly relaxes. Such a fact is shown in such a structural change that ΔHm1 decreases and half-value width at Tm1 increases while Δn doesn't change almost at all by the heat treatment. The processing time is too short to change the molecular weight. Reduced crystallinity generally causes a great reduction of mechanical characteristics. Although the strength and elastic modulus decrease without increasing in our heat treatment, the strength and elastic modulus are kept at a high level as maintaining high melting point (Tm1) and heat resistance to maintain the high molecular weight and orientation. The peak temperature of tan δ becomes high by high-temperature heat treatment and the peak value rises. The crystallinity is decreased by the heat treatment, so that the peak value rises and abrasion resistance improves. The peak temperature becomes high as a result that peaks of amorphous material are increased by crystal melting. Namely, the abrasion resistance is low, because the peak temperature is low and the crystallinity is high in a condition of performing no heat treatment at a high temperature.

EXAMPLES

Herein after, our invention will be explained with Examples. Each characteristic value has been determined by the following method.

A. Heat Characteristics (Tm1, Tm2, Tm1 Peak Half-Value Width, ΔHm1, ΔHm2)

Differential calorimetry is carried out by DSC 2920 made by TA Instruments Corporation to determine temperature of endothermic peak temperature Tm1 [° C.] under the condition of heating from 50° C. at temperature elevation rate of 20° C./min so that the heat of melting ΔHm1 [J/g] at Tm1 is determined. Maintaining temperature of Tm1+20° C. for five minutes after determination of Tm1, cooling is carried out down to 50° C. and then endothermic peak temperature Tm2 is determined under the condition of heating again at temperature elevation rate of 20° C./min so that the heat of melting (ΔHm2) [J/g] at Tm2 is determined. Fibers and resins are subject to the same measurement. Thus determined Tm2 is regarded as a melting point for the measurement of resins.

B. Weight Average Molecular Weight in Terms of Polystyrene (Molecular Weight)

Using a mixed solvent of pentafluoro phenol/chloroform=35/65 (weight ratio) as solvent, a sample for GPC measurement is prepared by dissolving to make the liquid crystal polyester have a concentration of 0.04 to 0.08 weight/volume %. When insoluble substance remains even after leaving at room temperature for 24 hours, the sample is left for additional 24 hours to collect the supernatant as a measurement sample. The sample is subject to a measurement using a GPC measurement apparatus made by Waters Corporation to determine weight average molecular weight (Mw) in terms of polystyrene.

Column: Shodex K-806M; two pieces, K-802; one piece Detector: Differential refractive index detector RI

Temperature: 23±2° C.

Flow rate: 0.8 mL/min Injection amount: 2004

C. Total Fineness, Single Fiber Fineness

A hank of fiber of 100 m is sampled with a sizing reel and then the weight [g] is multiplied at 1,000 times so that 3 times of measurements are carried out per 1 level to calculate an average value as a fiber fineness [dtex]. The calculation result is divided by the filament number to obtain a quotient as single fiber fineness [dtex].

D. Strength, Elongation, Elastic Modulus, Strength Fluctuation

Based on the method described in JIS L1013:2010 in condition of sample length 100 mm and tensile velocity 50 mm/min, 10 times of measurements per 1 level are carried out using Tensilon UCT-100 produced by Orientech Corporation to calculate an average value as strength [cN], elongation [%] and elastic modulus [cN/dtex]. Here, the elastic modulus means an initial tensile resistance degree. The strength fluctuation is calculated by the following formula using the greater absolute values of difference between the maximum or minimum value and the average value of 10 times of strength measurements.

Strength fluctuation [%]={(|maximum or minimum value−average value|/average value)×100}

E. Birefringence Index (Δn)

Using a polarization microscope (BH-2 made by Olympus Corporation), 5 times of measurements are carried out per 1 level of sample by the compensator method to calculate an average value.

F. Loss Tangent (Tan δ)

The peak temperature and peak value of loss tangent (tan δ) are determined by measuring the dynamic viscoelasticity from 60° C. to 210° C. with VIBRON DDV-II-EP made by Orientec Corporation under condition of frequency 110 Hz, initial load 0.13 cN/dtex, temperature elevation rate 3° C./m. When any peaks are not clearly observed, the maximum value of tan δ is regarded as a peak value and its temperature is regarded as a peak temperature in temperature elevation measurement. Namely, 60° C. or 210° C. is a peak temperature when no peak is clearly observed. When a plurality of peaks are observed, the maximum value is regarded as a peak value. When the peak top value continues for a certain range of temperature, the average value of the temperature is regarded as a peak temperature.

G. Oil Adhesion Rate to Fiber Weight

A sample of 100 mg or more of fibers is dried at 60° C. for 10 min and its dry weight (W0) is measured. The fiber is immersed in 2.0 wt % sodium dodecyl benzene sulphonic acid solution containing water of which weight is as 100 times or more as the fiber weight, and then subject to ultrasonic cleaning at room temperature for 20 min. The cleaned fiber is washed with water and dried at 60° C. for 10 min and its dry weight (W1) is measured. The oil adhesion rate is calculated by the following formula.

(Adhesion rate [wt %])=(W0−W1)×100/W1

H. Abrasion Resistance C

Fiber applied with load of 1.23 cN/dtex is hung vertically. A ceramic rod guide (made by Yuasa Itomichi Kogyo Corporation, Material; YM-99C) having diameter of 4 mm is pushed onto the fiber at a contact angle of 2.7° in a direction perpendicular to the fiber. The fiber is scratched by the guide in a fiber axial direction at stroke length of 30 mm and stroke speed of 600 times/min and is observed with a stereo microscope every 30 sec. The time period, until white powder or fibril is observed on the rod guide or the fiber surface, is measured to determine the abrasion resistance C by averaging the 5 times of measurement results except for maximum and minimum values among 7 times of measurements. When neither the white power nor the fibril is observed after scratching for 360 sec, the time period is regarded as 360 sec.

I. Thermal Deformation at High Temperature (Dry-Heat Dimensional Change Rate)

The dry-heat hank dimensional change rate determined according to the method described in JIS L1013:2010 is regarded as a thermal deformation at high temperature. The measurement condition is such that load of 3.0 cN/dtex is applied to measure a hank length while the treatment is carried out at 150° C. for 5 min. The load is the same as the one to be subject to the dry-heat treatment. The thermal deformation is calculated by the following formula.

(Thermal deformation rate [%])=(L1−L0)×100/L0

L0: hank length [cm] before dry-heat treatment L1: hank length [cm] after dry-heat treatment

J. Yarn Breakage in Heat-Treatment Process

From the number of yarn-breakage times and the treated fiber length in the heat-treatment process, the yarn-breakage times per 1,000,000 m is calculated by the following formula. The treated fiber length is length corresponding to one solid-phase polymerization package in Examples 1-8 and Comparative Examples 1-6 while the length is 5,000,000 m in Examples 9-11 and Reference Example 3.

(Yarn breakage [times/1,000,000 m]=(the number of yarn-breakage [times]×100/(treated fiber length [10,000 m])

L. Yarn-Making Property

The number of yarn-breakage times is measured when 500,000 m of fiber is wound in melt spinning process to determine the yarn-making property according to the following standard. Since the less the yarn breakage is the better the yarn-making property is, it is industrially preferable that the number of yarn breakage times is 2 or less.

 (Excellent): 0 times ∘ (Good): 1-2 times Δ (Acceptable): 3-4 times × (Bad): 5 times or more

Reference Example 1

p-hydroxy benzoic acid of 870 parts by weight, 4,4′-dihydroxy biphenyl of 327 parts by weight, hydroquinone of 89 parts by weight, terephthalic acid of 292 parts by weight, isophthalic acid of 157 parts by weight and acetic anhydride of 1,460 parts by weight (1.10 equivalent of the sum of phenolic hydride group) were mixed in a reaction vessel of 5 L with an agitating blade and a distillation tube, and after temperature was elevated from room temperature to 145° C. by 30 min while agitated under nitrogen gas atmosphere, it was reacted at 145° C. for 2 hours. Thereafter, the temperature was elevated to 335° C. by 4 hours. The polymerization temperature was kept at 335° C., the pressure was reduced down to 133 Pa for 1.5 hours, and further the reaction was continued for 40 min, and at the time when the torque reached 28 kgcm, the condensation polymerization was completed. Next, inside of the reaction vessel was pressurized at 0.1 MPa, the polymer was discharged as strand-like material through a spinneret having one circular discharge port having diameter of 10 mm, and it was pelletized by a cutter. Composition of thus obtained liquid crystal polyester, melting point and molecular weight are shown in Table 1.

Reference Example 2

p-hydroxy benzoic acid of 907 parts by weight, 6-hydroxy-2-naphthoic acid of 457 parts by weight and acetic anhydride of 946 parts by weight (1.03 mol equivalent of the sum of phenolic hydride group) were mixed in a reaction vessel with an agitating blade and a distillation tube, and after temperature was elevated from room temperature to 145° C. by 30 min while agitated under nitrogen gas atmosphere, it was reacted at 145° C. for 2 hours. Thereafter, the temperature was elevated to 325° C. by 4 hours. The polymerization temperature was kept at 325° C., the pressure was reduced down to 133 Pa by 1.5 hours, and further the reaction was continued for 20 min, and at the time when the torque reached a predetermined level, the condensation polymerization was completed. Next, inside of the reaction vessel was pressurized at 0.1 MPa, the polymer was discharged as strand-like material through a spinneret having one circular discharge port with diameter of 10 mm, and it was pelletized by a cutter. Composition of thus obtained liquid crystal polyester, melting point and molecular weight are shown in Table 1.

TABLE 1 Reference Reference Example 1 Example 2 p-hydroxybenzoate unit mol % 54 73 4,4′-dihydroxy biphenyl unit mol % 16 0 Hydroquinone unit mol % 7 0 Terephthalic acid unit mol % 15 0 Isophthalic acid unit mol % 8 0 6-hydroxy-2-naphthoic acid unit mol % 0 27 Liquid crystal Melting point ° C. 320 283 polyester properties Weight average ×10,000 10.4 23.0 molecular weight

Example 1

Using the liquid crystal polyester of Reference Example 1, after vacuum drying was carried out at 160° C. for 12 hours, it was melt extruded by a single-screw extruder of φ15 mm made by Osaka Seiki Kosaku Corporation, and the polymer was supplied to a spinning pack while metered by a gear pump. In the spinning pack, the polymer was filtered using a metal nonwoven fabric filter, and the polymer was discharged in the condition shown in Table 2. The introduction hole positioned right above the hole of the spinneret is straight shaped hole while the introduction hole and the spinneret hole are connected with a tapered portion. The discharged polymer was cooled and solidified from the outer side of the yarn by an annular cooling air wind after passing through the heat retention region of 40 mm, and thereafter, a spinning oil solution primarily constituting fatty acid ester compound was added, and all filaments were wound to the first godet roll at a spinning velocity shown in Table 2. After this was passed through the second godet roll at the same velocity, all filaments except for one were sucked by a suction gun, and the remaining one filament having the filament number 1 was taken up into a pirn form via a dancer arm using a pirn winder (EFT type take-up winder produced by Kamitsu Seisakusho Corporation, no contact roller contacting with a take-up package). During the take-up of 500,000 m, yarn breakage didn't occur and the yarn-making property was good. Spun yarn properties are shown in Table 2. Besides, no peak was clearly observed while tan δ monotonically increased with temperature elevation in the measurement with raw yarn of spinning. Therefore, the peak temperature defined in the specification was 210° C. and the peak value was 0.067.

TABLE 2 Example 1 Example 4 Example 5 Example 7 Example 8 Liquid crystal polyester polymer Reference Reference Reference Reference Reference Example 1 Example 1 Example 1 Example 1 Example 2 Melt Spinning temperature ° C. 345 345 345 345 325 spinning Discharge rate g/min 2.4 3.1 1.9 3.3 1.4 conditions Spinneret opening mm 0.13 0.13 0.13 0.13 0.20 diameter Land length mm 0.26 0.26 0.26 0.26 0.30 L/D — 2.0 2.0 2.0 2.0 1.5 Opening number units 4 4 4 5 4 Yarn velocity m/min 1000 600 1200 1500 600 Yarn draft — 27 12 40 36 63 Spinnability — (Excellent) (Excellent) ◯(Good) ◯(Good) ◯(Good) Spun Weight average x10,000 10.2 10.2 10.2 10.0 8.8 yarn molecular weight properties Total fineness dtex 6.0 13.0 4.0 22.0 6.0 Filament number pieces 1 1 1 5 1 Single fiber fineness dtex 6.0 13.0 4.0 4.4 6.0 Tm1 ° C. 299 298 300 300 285 Strength cN/dtex 6.4 6.1 6.2 6.6 8.7 Elongation % 1.5 1.5 1.4 1.4 2.1 Elastic modulus cN/dtex 531 514 545 588 547

The fiber was rolled back from this spun fiber package by SSP-MV type rewinder (contact length of 200 mm, the number of winding of 8.7, taper angle of 45°) made by Kamitsu Seisakusho Corporation. The spun fiber was unraveled in a vertical direction (direction perpendicular to the fiber-rounding direction). Without using a speed-regulating roller, oil solution for solid-phase polymerization was supplied by an oiling roller having a stainless-steel roll with satin-finished surface. The oil solution for solid-phase polymerization employed was 6.0 wt % phosphate compound (B) of phosphate compound (B1) shown in Chemical formula (4) in which 1.0 wt % inorganic particle (A) of talc SG-2000 (made by NIPPON TALC Co., Ltd.) was dispersed.

Kevlar felt (areal weight: 280 g/m2, thickness: 1.5 mm) rolled on a stainless-steel bobbin with holes was used as a core member for the roll-back while the surface pressure was set to 100 gf. The oil adhesion rate to the rolled-back fiber of oil solution for solid-phase polymerization as well as roll-back conditions are shown in Table 3. Next, the stainless-steel bobbin with holes was detached from the rolled-back package, solid-phase polymerization was carried out in a condition of package where the fiber was taken up on the Kevlar felt. The solid-phase polymerization was carried out with a closed type oven to elevate temperature from room temperature to 240° C. by about 30 min and then keep the temperature at 240° C. for 3 hours. Again, the temperature is elevated to the highest temperature shown in Table 3 by 4° C./hour and kept the retention time shown in Table 3. In the atmosphere of oven, dehumidified nitrogen was supplied at a flow rate of 20 NL/min and discharged from an exhaust port to prevent the inner pressure from becoming too high. Fiber properties after solid-phase polymerization are shown in Table 3. The abrasion resistance was poor since abrasion resistance C of fiber after solid-phase polymerization was 30 sec only.

TABLE 3 Example 1 Example 4 Example 5 Example 7 Example 8 Roll-back Roll-back velocity m/min 400 400 400 400 400 condition Winding tension cN/dtex 0.16 0.10 0.30 0.10 0.16 Winding density g/cc 0.5 0.3 0.6 0.3 0.5 Winding quantity x10,000 (m) 50.0 25.0 10.0 10.0 10.0 Oil adhesion rate wt % 15.0 12.2 18.6 20.0 15.2 (a + b) of oil solution for solid-phase polymerization Solid-phase Highest temperature ° C. 290 290 290 290 290 polymerization Retention time to reach hr 20 20 20 20 20 highest temperature Fiber Weight average x10,000 39.6 38.2 41.2 40.8 37.0 properties molecular weight after solid-phase Total fineness dtex 7 14 4 24 6 polymerization Filament number pieces 1 1 1 5 1 Single fiber fineness dtex 6.5 13.9 4.3 4.8 6.5 Strength cN/dtex 22.6 20.1 21.7 21.3 21.1 Elongation % 2.8 2.6 2.6 2.6 3.1 Elastic modulus cN/dtex 988 952 1072 1014 820 Tm1 ° C. 332 332 334 333 318 ΔHm1 J/g 9.0 8.6 10.2 9.2 11.4 Peak half-value width ° C. 10 12 8 10 7 at Tm1 Tm2 ° C. 333 332 334 331 316 ΔHm2 J/g 1.2 1.2 1.1 1.1 0.9 Oil adhesion per wt % 7.8 7.0 8.2 9.0 7.5 fiber weight

Finally, fiber was unraveled from the package after solid-phase polymerization and successively subject to a high-temperature non-contact heat treatment. The package after solid-phase polymerization was attached to a free roll creel (having a shaft and bearings to freely rotate outer layer, without brakes and drive sources) and therefrom the yarn was drawn out in a lateral direction (fiber-rounding direction). Successively the fiber was dipped in a bath (with no guides to contact fiber inside) of bath length of 150 cm (contact length of 150 cm) provided with slits at both ends to remove oil solution by washing. The washing liquid containing 0.2 wt % nonionic-anionic surfactant (Gran Up US-30 made by Sanyo Chemical Industries Corporation) controlled at 50° C. with an external tank was supplied into a tank by a pump. The liquid was supplied into the tank through a pipe having holes provided at intervals of 5 cm in the tank to generate a liquid flow through the pipe in the tank. The washing liquid overflowed from slits and holes for adjusting liquid level was returned to the external tank in a certain mechanism.

Successively the fiber was dipped in a bath (with no guides to contact fiber inside) of bath length of 23 cm (contact length of 23 cm) provided with slits at both ends to be rinsed with water at 50° C. The washed fiber was passed through a bearing roller guide and was contacted to air flow to blow off the water to be removed, and then was passed through the first roller having a separate roller at 200 m/min. The creel is a free roll, to which tension is applied to unravel the solid-phase polymerized package to feed the fiber.

The fiber which had passed through the roller was fed between heated slit heaters and was subject to high-temperature heat treatment under the conditions shown in Table 4. The slit heaters were not provided with guides inside while the heater didn't contact the fiber. The fiber which had passed through the heater was passed through the second roller having a separate roller. The yarn velocity before heat treatment represents a surface velocity of the first roller while the yarn velocity after heat treatment represents a surface velocity of the second roller. A finishing oil solution primarily consisting of fatty acid polyester compound is added to the fiber which had passed through the second roller as using an oiling roller made of ceramic, and was taken up into a pirn form with EFT type bobbin traverse winder (made by Kamitsu Seisakusho Corporation). Fiber properties after high-temperature heat treatment are shown in Table 4. An of the liquid crystal polyester fiber was 0.35 representing a high orientation.

Because the fiber obtained in Example 1 achieved both high abrasion resistance and low thermal deformation rate, it is expected that processability could be improved at a higher processing, faults could be reduced and thermal deformation could be suppressed in processing at a high temperature.

TABLE 4 Comparative Comparative Comparative Comparative Example 1 Example 1 Example 2 Example 3 Example 2 Example 3 Example 4 Fiber after solid-phase polymerization Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 Example 1 High- Heater temperature ° C. 480 480 480 510 480 480 480 temperature Heater length mm 1000 1000 1000 1000 1000 1000 1000 heat Yarn velocity before m/min 198 200 190 190 199 195 193 treatment heat-treatment Yarn velocity after m/min 200 200 200 200 200 200 200 heat-treatment Stretch rate % 1.0 0.0 5.0 5.0 0.5 2.5 3.5 Treatment time sec 0.30 0.30 0.30 0.30 0.30 0.30 0.30 Running tension gf 0.5 0.3 1.8 0.3 0.4 0.9 1.2 Running stress cN/dtex 0.08 0.06 0.30 0.05 0.08 0.14 0.20 Yarn breakage times/ 0 12 50 10 0 10 20 million meters Fiber Weight average x10,000 39.3 39.2 Sampling 39.2 39.3 39.2 39.2 properties molecular weight impossible after high- Total fineness dtex 6.0 6.0 6.0 6.0 6.0 6.0 temperature Filament number pieces 1 1 1 1 1 1 heat treatment Single fiber fineness dtex 6.0 8.0 6.0 6.1 6.1 6.1 Strength cN/dtex 18.8 17.9 17.6 18.4 19.2 19.3 Elongation % 2.9 2.9 3.0 2.9 2.8 2.8 Elastic modulus cN/dtex 785 745 738 761 832 851 Tm1 ° C. 323 322 322 323 323 320 ΔHm1 J/g 0.8 0.6 0.6 0.7 1.5 5.1 Peak half-value width ° C. 25 31 35 28 21 15 at Tm1 Tm2 ° C. 333 333 334 333 333 333 ΔHm2 J/g 1.1 1.0 1.0 1.1 1.1 1.2 Abrasion resistance sec 360 360 360 360 180 48 tan δ peak value — 0.077 0.091 0.092 0.85 0.065 0.059 tan δ peak temperature ° C. 148 145 145 144 142 139 Oil adhesion per wt % 0.8 0.8 0.8 0.8 0.8 0.8 fiber weight Thermal deformation rate % 0.4 1.1 1.2 0.7 0.3 0.2 at hight temperature Comparative Comparative Example 4 Example 5 Example 6 Example 7 Example 8 Example 5 Example 4 Fiber after solid-phase polymerization Example 4 Example 5 Example 1 Example 7 Example 8 Example 1 Example 8 High- Heater temperature ° C. 500 460 500 520 480 No heat- No heat- temperature Heater length mm 1000 1000 1000 1000 1000 treatment treatment heat Yarn velocity before m/min 198 198 396 199 198 carried out carried out treatment heat-treatment Yarn velocity after m/min 200 200 400 200 200 heat-treatment Stretch rate % 1.0 1.0 1.0 0.5 1.0 Treatment time sec 0.30 0.30 0.15 0.13 0.30 Running tension gf 0.5 0.4 0.7 0.6 0.5 Running stress cN/dtex 0.08 0.03 0.11 0.08 0.08 Yarn breakage times/ 0 10 0 10 10 million meters Fiber Weight average x10,000 38.0 40.8 39.2 40.1 37.1 39.6 36.9 properties molecular weight after high- Total fineness dtex 12.9 4.0 6.0 21.8 6.8 6.0 6.0 temperature Filament number pieces 1 1 1 5 1 1 1 heat treatment Single fiber fineness dtex 12.9 4.0 6.0 4.4

.8 6.0 6.0 Strength cN/dtex 17.1 18.0 18.6 16.3 18.5 21.1 20.1 Elongation % 2.8 2.8 2.9 2.6 3.0 2.7 3.0 Elastic modulus cN/dtex 709 765 764 704 749 905 757 Tm1 ° C. 322 322 322 323 306 332 318 ΔHm1 J/g 0.6 0.7 0.8 0.7 5.8 9.0 11.4 Peak half-value width ° C. 33 26 26 27 16 10 7 at Tm1 Tm2 ° C. 333 334 333 332 318 333 316 ΔHm2 J/g 1.1 0.9 1.2 1.1 0.9 1.2 0.9 Abrasion resistance sec 240 12.0 240 240 80 30 30 tan δ peak value — 0.072 0.077 0.075 0.080 0.085 0.041 0.064 tan δ peak temperature ° C. 143 144 144 1.45 110 120 85 Oil adhesion per wt % 0.0 1.0 1.0 1.6 0.9 0.9 0.9 fiber weight Thermal deformation rate % 0.5 0.6 0.5 0.8 0.3 0.1 0.1 at high temperature

indicates data missing or illegible when filed

Comparative Examples 1-4, Examples 2 and 3

The effect of stretch rate in a high-temperature heat treatment was evaluated. The solid-phase polymerized yarn obtained in Example 1 was heat treated at a high temperature by the same method as Example 1 except that the heat-treatment temperature and stretch rate were changed according to Table 4. The stretch rate was 5.0% in Comparative Example 2, in which the yarn breakage occurred right after the heat treatment. The yarn breakage occurred twice during the treatment of 40,000 m to cancel the test because a sample of 30,000 m or more was not obtained. Properties of obtained fiber are shown in Table 4. The table shows that obtained fiber can achieve both excellent abrasion resistance and low thermal deformation rate with less yarn breakage when the stretch rate is 0.1% or more and less than 3.0%. The stretch rate was low in Comparative Example 1, in which relatively many times of yarn breakage occurred in heat treatment while the tan δ peak value and thermal deformation rate were high. The stretch rate was 5.0% in Comparative Example 3, in which the tan δ peak value increased and thermal deformation rate was high because the temperature was increased to suppress yarn breakage. The stretch rate was high in Comparative Example 4, in which the abrasion resistance was poor in spite of low tan δ peak value.

Examples 4 and 5

The effect of single fiber fineness was evaluated. The melt spinning was carried out by the same method as Example 1 except that the discharge rate and spinning velocity were changed according to Table 2. The single fiber fineness was small in Example 5, in which the yarn breakage occurred once although spinnability was good. Properties of obtained fiber are shown in Table 2. Next, the solid-phase polymerization was carried out by the same roll-back method as Example 1, except that the winding condition (quantity, tension and density) were changed according to Table 3. Properties of obtained fiber after solid-phase polymerization are shown in Table 3. The high-temperature heat treatment was carried out by the same method as Example 1, except that the heat-treatment temperature was changed according to Table 4. The single fiber fineness was small in Example 5, in which the yarn breakage occurred once during the treatment of 100,000 m although processability had almost no problem. Properties of obtained fiber are shown in Table 4. The table shows that obtained fiber can achieve both excellent abrasion resistance and low thermal deformation rate even under various single fiber fineness when the stretch rate is 0.1% or more and less than 3.0% under controlled heat-treatment temperature.

Example 6

The effect of heat-treatment velocity was evaluated. The solid-phase polymerized yarn obtained in Example 1 was heat treated at a high temperature by the same method as Example 1, except that the heat-treatment temperature and stretch rate were changed according to Table 4. Properties of obtained fiber are shown in Table 4. The table shows that obtained fiber can achieve both excellent abrasion resistance and low thermal deformation rate with less yarn breakage even under various velocities of treatment when the stretch rate is 0.1% or more and less than 3.0% under controlled heat-treatment temperature.

Example 7

The effect of the number of filaments was evaluated. The melt spinning was carried out by the same method as Example 1, except that the discharge rate, spinneret opening number and spinning velocity were changed according to Table 2 while discharged filaments were converged to make a multifilament. The yarn breakage occurred once although spinnability had no problem. Properties of obtained fiber are shown in Table 2. Next, the solid-phase polymerization was carried out by the same roll-back method as Example 1 except that the winding quantity was changed according to Table 3. Properties of obtained fiber after solid-phase polymerization are shown in Table 3. The high-temperature heat treatment was carried out by the same method as Example 1, except that the heat-treatment temperature and stretch rate were changed according to Table 4. The yarn breakage occurred once during the treatment of 100,000 m although processability had almost no problem. Properties of obtained fiber are shown in Table 4. The table shows that obtained fiber can achieve both excellent abrasion resistance and low thermal deformation rate even with multifilament when the stretch rate is 0.1% or more and less than 3.0% under controlled heat-treatment temperature.

Example 8

The effect of polymer composition was evaluated. The polymer obtained in Reference Example 2 was melt spun by the same method as Example 1, except that the spinneret opening number, land length, discharge rate and spinning velocity were changed according to Table 2. The yarn breakage occurred once although spinnability had no problem. Properties of obtained fiber are shown in Table 2. Next, the solid-phase polymerization was carried out by the same roll-back method as Example 1, except that the winding quantity was changed according to Table 3. Properties of obtained fiber after solid-phase polymerization are shown in Table 3. Next, the high-temperature heat treatment was carried out by the same method as Example 1. The yarn breakage occurred once during the treatment of 100,000 m although processability had almost no problem. Properties of obtained fiber are shown in Table 4. The table shows that obtained fiber can achieve both good abrasion resistance and low thermal deformation rate even under various composition when the stretch rate is 0.1% or more and less than 3.0% under controlled heat-treatment temperature.

Comparative Examples 5 and 6

The effect of high-temperature heat treatment was evaluated. Using the solid-phase polymerized yarn obtained in Examples 1 and 8, the fiber was fed and taken up by the same heat-treatment method as Examples 1 and 8, except that the rollers before and after the heater were run at 200 m/min at room temperature while the heater was not operated. Namely, the solid-phase polymerized fiber was unraveled and washed to be rolled back without heat treatment. Properties of obtained fiber are shown in Table 4. The table shows that the high-temperature heat treatment was not carried out to make the abrasion resistance low although thermal deformation rate was low. The table also shows that both good abrasion resistance and low thermal deformation rate cannot be achieved in a case such as Comparative Example 5 in which the tan δ peak value was low and Comparative Example 6 in which the peak temperature was low.

Example 9, Reference Example 3

The effect of providing a guide at the exit of heating region was determined through a long-run evaluation. Namely, the solid-phase polymerized yarn of 5,000,000 m was subject to high-temperature heat treatment to evaluate the yarn breakage in particular. Using the solid-phase polymerized yarn obtained in Example 1, the high-temperature heat treatment was carried out by the same method as Example 1, except that two pieces of hard chrome-plated satin-finished metal rod guides (made by Yuasa Itomichi Kogyo Corporation, Rzjis=2-4) having diameter of 3.8 mm were provided at the exit of heater for heat treatment according to Table 5. The treatment length was 5,000,000 m corresponding to 10 pieces of solid-phase polymerized yarn (Example 9). The high-temperature heat treatment of 5,000,000 m was carried out under the same condition as Example 1 without providing a guide (Reference Example 3). Reference example 3 and Example 1 have a difference of treatment length only. Properties of obtained fiber are shown in Table 5. The table shows that Example 9 is excellent in running stability with less yarn breakage relative to Reference Example 3. The properties show small strength fluctuation rates representing less fluctuation. We presume the stable treatment contributed to a smaller variation in the example because the strength, elongation and elastic modulus were slightly higher than Reference Example 3. Thus provided guide at the exit of heating region can regulate the yarn route to suppress yarn breakage.

TABLE 5 Reference Example 9 Example 3 Example 10 Example 11 Fiber after solid-phase polymerization Example 1 Example 1 Example 1 Example 1 High- Heater temperature ° C. 480 480 480 480 temperature Heater length mm 1000 1000 1000 1000 heat Yarn velocity before m/min 198 198 195 195 treatment heat treatment Yarn velocity after m/min 200 200 200 200 heat treatment Stretch rate % 1.0 1.0 2.5 2.5 Treatment time sec 0.30 0.30 0.30 0.30 Guide setting position cm 5 No guide 3 50 Running tension at gf 0.4 — Unmeasurable 0.9 heating region side (T1) Running tension at gf 0.5 0.5 0.9 0.9 rewind side (T2) T2/T1 — 1.25 — — 1.00 Yarn breakage caused by times/ 0.2 0.8 2.2 8.0 heat treatment of million 5,000,000m meters Fiber Weight average x10,000 39.3 39.3 39.2 39.2 properties molecular weight after Tolal fineness dtex 6.0 6.0 6.0 6.0 high- Filament number pieces 1 1 1 1 temperature Single fiber fineness dtex 6.0 6.0 6.1 6.1 heat Strength cN/dtex 18.9 18.6 19.3 19.1 treatment Strength variation rate % 4 8 5 5 Elongation % 2.9 2.8 2.9 2.8 Elastic modulus cN/dtex 788 776 851 816 Tm1 ° C. 322 323 324 323 ΔHm1 J/g 0.7 0.8 1.5 1.5 Peak half-value width ° C. 26 25 22 21 at Tm1 Tm2 ° C. 332 333 333 333 ΔHm2 J/g 1.1 1.1 1.1 1.1 Abrasion resistance sec 360 360 198 180 tan δ peak value — 0.078 0.077 0.067 0.065 tan δ peak temperature ° C. 144 143 143 142 Oil adhesion per wt % 0.8 0.8 0.8 0.8 fiber weight Thermal deformation rate % 0.4 0.4 0.3 0.3 at high temperature

Examples 10 and 11

The effect of position for setting a guide at the exit of heating region was determined through a long-run evaluation. The high-temperature heat treatment was carried out by the same method as Example 9, except that the guide setting position was changed according to Table 5. Examples 10 and 11 have the same stretch rate as Example 3, and have different guide setting positions and treatment lengths from Example 3. Properties of obtained fiber are shown in Table 5. T1 wasn't able to be measured since the guide setting position was close to the heating region (heater) in Example 10. The yarn breakage was reduced in Example 10 better than Example 3 in spite of long treatment length. The number of yarn breakage times was reduced even in Example 11 better than Example 3. Thus the position distant from the heating region by 1 cm or more and 50 cm or less can suppress yarn breakage. 

1. A liquid crystal polyester fiber having: a peak half-value width of 15° C. or more at an endothermic peak (Tm1) observed by a differential calorimetry under a temperature elevation condition of 20° C./min from 50° C.; a weight-average molecular weight in terms of polystyrene of 250,000 or more and 2,000,000 or less; a peak temperature of a loss tangent (tan δ) of 100° C. or more and 200° C. or less; and a peak value of the loss tangent (tan δ) of 0.060 or more and 0.090 or less.
 2. The liquid crystal polyester fiber according to 1, wherein the fiber has a strength fluctuation rate of 10% or less.
 3. A mesh fabric comprising the liquid crystal polyester fiber according to claim
 1. 4. A producing method of a melt-spun liquid crystal polyester fiber characterized in that a liquid crystal polyester fiber made by a melt spinning is polymerized in a solid phase and then heated at a temperature of an endothermic peak (Tm1)+50° C. or more by a stretch rate of 0.1% or more and less than 3.0%, wherein the endothermic peak is observed by a differential calorimetry under a temperature elevation condition of 20° C./min from 50° C.
 5. The producing method according to claim 4, wherein the heated fiber is taken up under a yarn route regulation with yarn route guide in a range of 1 cm or more and 50 cm or less from an exit portion of a heating region. 